A number of prior art products have been made which are conductive and flexible. These products include materials made by drying and polymerizing dispersions of conductive carbon in a binder of elastomer. In a number of the prior art products, the carbon is wetted and ground to a fine paste which is mixed with a polymeric binder. The resulting composition is dried and cured to form a conductive, flexible material. The conductive carbon is ground to submicroscopic size using higher shear methods. The bulk of carbon is reduced to a size below 0.1 micrometers. Such finely ground carbon appears as a brown haze in the microscope. The carbon "grind" prepared by conventional mixing is considered unsatisfactory because the carbon particles are intimately adsorbed to the binder and conductivity is achieved only with an excess of carbon resulting in a randomly mixed bulk composition of poor pressure-conductive properties. In an alternative prior art process, the carbon particles are dispersed dry in a semi-solid prepolymer or monomer under high shear by milling action, and the mixture is cured and solidified to form a conductive rubber which shows conductivity but poor pressure-conductive characteristics.
The prior art conductive rubbers require a high carbon loading and sufficient binder to maintain an integral structure of the conductive rubber. Silicon rubber with dispersed conductive carbon is an example of such a conductive rubber. Because of the required high carbon loading, conventional conductive rubbers do not possess strong integrity and are cast into thin sheets. It is especially difficult to coat and difficult to obtain pressure sensitive coatings with prior art conductive rubbers.
Most of the conventional conductive rubbers upon the application of pressure or mechanical force do not exhibit a significant, if any, change in electrical resistance. Such material is treated and used as a fixed resistance material. Expensive shaping and specially designed electrodes are required to produce pressure sensitive electro-conductive devices from conventional conductive rubber. Thus, direct application of the conventional conductive rubbers does not result in a useful force discriminating sensor which can sense beyond opened/closed positions. Moreover, the conventional conductive rubbers cannot be used in touch feed-back systems and directly monitored switches which indicate closed circuits with open switches. Where a surface is roughened and formed into irregular geometry, the function of sensitivity with pressure is limited and difficult to control.
My U.S. Pat. No. 4,054,540 is directed to an electric resistant element sensitive to pressure comprising a substantially discontinuous phase of metallic conducting particles in a matrix of a cured elastomeric resin. The metallic conducting particles are coated with a deformable, semi-conducting compound. The element has a high loading of metal conducting particles to resin of from 75:100 to 110:100 by weight.
My U.S. Pat. No. 4,120,828 is directed to finely divided metal particles coated with a deformable, electrically semi-conductive compound. The particles can be employed in an electric resistant element which is sensitive to pressure.
U.S. Pat. No. 4,258,100 is directed to a pressure sensitive electric-conductive sheet material comprising at least one layer of rubbery elastic material and an adhesive layer disposed on at least one of the surfaces of the sheet. Both layers having substantially uniform distributed fine particles of electric conductive metal. The particle size of the fine metal particles is from 10-1000 mesh and the loading of the sheet material of metal particles to the rubbery elastic material is 10:100 to 800:100 by weight.